Molecular Spectra and Optical Relaxation of Oxazine Dyes Nile Blue and Cresyl Violet.

Using fluorescence-line-narrowing and hole-burning techniques I obtain molecular spectra of the oxazine dye nile blue at 5 K under a wide range of experimental conditions. These data are analyzed by constructing a single site molecular lineshape function and the corresponding 0-0 transition frequency distribution which, taken together, enable me to explain the various results. I also perform time-delayed-four-wave-mixing experiments (TDFWM) on nile blue and cresyl violet at 5 K using incoherent laser radiation. The response depends dramatically on the bandwidth and frequency of the radiation source. Fast (femtosecond) and slow (picosecond) processes, variable peak shifts, and type I and II quantum beats are observed in varying degrees according to the character of the laser excitation. I calculate the TDFWM response of nile blue on the basis of the single site lineshape function and the inhomogeneous distribution function with information from fluorescence-line-narrowing and hole-burning experiments and find that I can explain the wide range of behavior observed here. TDFWM experiments are also carried out on nile blue and cresyl violet at room temperature, using an intense incoherent laser. The response contains a modulated component which bears no relation to the dyes' energy level structures. I explain the observed modulation as a result of a destructive interference between induced high order polarizations. This modulation is a consequence of irradiating the sample at high intensities and, in contrast to modulation arising from quantum beats, disappears when the intensity of the laser excitation is reduced.